The present disclosure relates generally to systems and methods for medical imaging and, in particular, to systems and methods for diffusion-weighted imaging (“DWI”).
Any nucleus that possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency), which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the gyromagnetic ratio γ of the nucleus). Nuclei which exhibit these phenomena are referred to herein as “spins.”
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a transient radiofrequency electromagnetic pulse (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides on signals that are emitted by the excited spins after the pulsed excitation signal B1 is terminated. Depending upon chemically and biologically determined variable parameters such as proton density, longitudinal relaxation time (“T1”) describing the recovery of Mz along the polarizing field, and transverse relaxation time (“T2”) describing the decay of Mt in the x-y plane, this nuclear magnetic resonance (“NMR”) phenomena is exploited to obtain image contrast and concentrations of chemical entities or metabolites using different measurement sequences and by changing imaging parameters.
When utilizing NMR to produce images and chemical spectra, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region to be imaged (region of interest) is scanned using a sequence of NMR measurement cycles that vary according to the particular localization method being used. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) which have the same direction as the polarizing field B0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified from the Larmor frequencies typical of the local field. The acquisition of the NMR signals is referred to as sampling k-space, and a scan is completed when sufficient NMR cycles are performed to fully or partially sample k-space. The resulting set of received NMR signals are digitized and processed to reconstruct the image using various reconstruction techniques.
To generate an MR anatomic image, gradient pulses are typically applied along the x, y and z-axis directions to localize the spins along the three spatial dimensions, and MR signals are acquired in the presence of one or more readout gradient pulses. An image depicting the spatial distribution of a particular nucleus in a region of interest of the object is then generated, using known post-processing techniques. Typically, the hydrogen nucleus (1H) is imaged, though other MR-detectable nuclei may also be used to generate images.
Diffusion-weighted imaging (“DWI”) is an important MM technique that is based on the measurement of random motion of water molecules in tissues and provides directionally dependent microstructural information across a wide range of spatial scales. DWI has been utilized for studying the anatomy of the brain, including neural architecture and brain connectivity, as well as various brain disorders, including Alzheimer's disease, schizophrenia, mild traumatic brain injury, and so forth. In particular, DWI has been widely used to estimate the apparent diffusion coefficient (“ADC”) in the brain, and is considered the clinical gold standard for detection of acute and chronic stroke. DWI has also demonstrated clinical value in the heart and liver, but sensitivity to macroscopic motion (especially cardiac and respiratory bulk motion) frequently contributes to large signal losses that confound measurements in these regions when using conventional methods.
Although some techniques have been developed to try to address macroscopic motion, these are not without shortcomings. For instance, synchronizing the DWI acquisition with physiologic motion has been one approach attempting to mitigate motion artifacts. Particularly in the liver, bulk motion artifacts can be partly reduced by implementing cardiac and respiratory triggering. However, this comes at a significant cost of increased acquisition time. In addition, triggering and respiratory motion compensation (via triggering, breath holds, or navigators) are insufficient to suppress bulk motion artifacts in the heart.
Therefore, there is a need for improved imaging systems and methods sensitive to diffusion that can be performed quickly and are insensitive to macroscopic motion artifacts.